Etching solution for wet chemical pyramidal texture etching of silicon surfaces

ABSTRACT

A new and improved etching solution and etching method provide wet chemical pyramidal texture etching of (100) silicon surfaces. A uniform and completely pyramidal texture etching of silicon surfaces is achieved with an etching solution including water, an alkaline reagent, and isopropanol together with an aqueous alkaline ethylene glycol solution.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of pending U.S. applicationSer. No. 09/272,022, filed on Mar. 18, 1999.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a method for the wet chemicalpyramidal texture etching of silicon surfaces.

[0003] A textured surface reduces the reflection of incident lightacross wide bands, thereby increasing the absorbed light intensity. Whenmanufacturing crystalline silicon solar cells, a systematic structuringof the silicon surface leads to an increase in solar cell performance.Such a wet chemical structural etching is not, however, restricted justto the processing and production of solar cells, but is also suitablefor optical and electrochemical detectors/sensors,biodetectors/biosensors, catalysts, electrodes and the like.

[0004] Known techniques for the wet chemical pyramidal structuraletching of (100)-oriented silicon surfaces use alkaline media, mainlywith solutions of alkali hydroxides, alkali carbonates, ammonia orcholine. Solutions of hydrazine or ethylene diamine/pyrocatechol canalso be used, but they are disadvantageous on account of their toxicity.The most common recipes comprise water, sodium or potassium hydroxideand alcohol. The alcohol component used is either ethylene glycol orisopropanol. Nevertheless, the known methods for the wet chemicalstructural etching of silicon merely relate, purely and simply, to thegeneration of a pyramidal (tetragonal) texture.

[0005] In terms of fine line metalization and the application of dopingresists or photoresist, however, a texture that exhibits small pyramidsis desirable. A method based on an etching solution that containsethylene glycol is described in EP 0 477 424 A1. The etching solutionrecipe used here contains water, potassium hydroxide, ethylene glycoland silicon. Oxygen is also added as a further component. After thesilicon surface has undergone wet chemical structural etching,reproducibly uniform pyramids are obtained by aerating the etchingsolution with oxygen. The pyramid height can be varied by the aeratingduration of the etching solution. Introducing oxygen over a longerperiod, i.e. greater oxidation of the glycol, results in smallerpyramids. In this way, pyramid sizes of ≦2 μm can be produced even interms of manufacturing engineering. But the disadvantage is that theetching solution based on ethylene glycol cannot be used ad hoc becauseit requires a preceding dissolution of silicon. Studies have shown thata subsequent rest phase of several hours proves to be beneficial forformation of smooth (111) faces. A simple addition of silicate solutiondoes not lead to success.

[0006] Methods which use an etching solution based on isopropanol areknown for example from U.S. Pat. No. 3,998,659, from “Uniform PyramidFormation on Alkaline-etched Polished Monocrystalline (100) SiliconWafers”, Bressers et al., Progress in Photovoltaics, Vol. 4, 435-438(1996), and from “Experimental Optimization of an Anisotropic EtchingProcess for Random Texturization of Silicon Solar Cells”, King et al.,IEEE 1991, 303-308. In contrast to the etching solution based onethylene glycol, an IPA-based etching solution can be used immediatelyfor the purpose of texture etching. This etching solution can be usedwith and without silicate. This type of solution nevertheless suffersfrom the drawback of a high rate of evaporation, on account of theisopropanol's boiling point of just 82° C. This means that problemsarise with regard to uniform etching and reproducibility of thepyramidal surface. In contrast to the etching solution based on ethyleneglycol, the IPA-based etching solution tends toward large pyramids.According to Bressers et al., small pyramids can be achieved by usingnitrogen to displace the dissolved oxygen during the etching process.But there are reports of laboratory experiments in which a 10 vol. %isopropanol etching solution, a nitrogen flow of 25 l/min and a processtemperature of 80° C. are used. An enormous consumption of isopropanolmust be expected for production application in tanks. This entails highcosts since the durability of the etching solution is low on account ofsuch a high volatilization of IPA. This simultaneously leads to highwaste-disposal costs for the spent chemicals.

[0007] As regards the industrial production of solar cells, it isimportant to guarantee a constantly good electrical quality of the solarcells produced in large quantities. Constant good quality in terms ofthe shape of the pyramid structure and a uniform structuring of thetotal silicon surface must be ensured. The ability to vary the pyramidsize within a certain range is also advantageous.

[0008] Neither the etching solution based on ethylene glycol nor theisopropanol-based solution are able, however, to meet these requirementssimultaneously to a satisfactory extent. As summarized above, theaqueous alkaline etching solution to which ethylene glycol is added isindeed able to produce pyramids with a height of ≦2 μm, but it cannot beused immediately. Yet precisely this is a drawback as regards industrialuse. Although the etching solution that contains isopropanol can, on theother hand, be used immediately for texture etching, it does tend towardlarge pyramids and proves to be disadvantageous as a result of theisopropanol's high rate of evaporation, since this impairs thereproducibility of a uniform pyramidal texture.

[0009] The object of the present invention is therefore to provide amethod for the wet chemical pyramidal texture etching of siliconsurfaces which is able—analogous to the etching solution based onethylene glycol—to generate reproducibly small (≦2 μm) pyramid sizes,with different sizes of pyramids being adjustable within a certain rangeor scatter, while simultaneously ensuring complete texturization of thesilicon surface and at the same time reducing costs.

SUMMARY OF THE INVENTION

[0010] In an embodiment, the present invention provides a new andimproved etching solution used for the pyramidal structuring of thesilicon surface which comprises both isopropanol and an aqueous alkalineethylene glycol solution. The aqueous alkaline ethylene glycol solutionis advantageously reacted with oxygen, as is known for example from EP 0477 424 A1. The greater the amount of oxygen reacted, the smaller theamount of ethylene glycol solution used in the etching solution. Theobject is surprisingly solved just by the simultaneous use ofisopropanol and ethylene glycol in the etching solution, which alsoentails the following advantages. At the start of etching, the use ofwater which is already preheated and is hence low in oxygen—as alsodescribed by Bressers et al.—has a supportive effect in terms of smallpyramids, but is not absolutely necessary because it can be compensatedby a corresponding correction in the ratio of glycol. Preheatedisopropanol—also described in Bressers et al.—is problematic for reasonsof production technology.

[0011] The etching solution according to the invention is capable oftexture immediately after production, i.e. an etching process can beperformed directly after the solution's production.

[0012] The etching result is also relatively insensitive to the exactcomposition of the etching solution, i.e. precise optimization of theratio of the individual dissolved components is not necessary, which isbeneficial particularly in terms of industrial application.

[0013] Another advantage is the good reproducibility of the etchingresult. At the same time, the pyramid size is adjustable, whilesimultaneously ensuring complete texture of the silicon surface.

[0014] A further advantage is obtained in that the present invention'setching solution does not suffer as much from the disadvantage of aconventional etching solution based just on isopropanol, i.e. aconsiderable rate of evaporation. This means that the etching solutionaccording to the invention can be used for etching over a much longerperiod than a conventional solution that is based on isopropanol alone.The etching solution therefore has to be replaced less frequently. Thislowers costs since downtime is decreased, thus reducing the amount ofwaste chemicals to be disposed of.

[0015] Other objects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the Drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 shows a scanning electron micrograph of the etched surfaceof a first exemplary embodiment;

[0017]FIG. 2 shows a scanning electron micrograph of the etched surfaceof a second exemplary embodiment;

[0018]FIG. 3 shows a scanning electron micrograph of the etched surfaceof a third exemplary embodiment; and

[0019]FIG. 4 shows a scanning electron micrograph of the etched surfaceof a fourth exemplary embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] The solution according to the invention for etching a firstexemplary embodiment is composed of 15 l water, 300 ml 50% sodiumhydroxide solution, 400 ml dissolved silicate, 600 ml isopropanol and 15ml aqueous alkaline ethylene glycol solution. The temperature of theetching solution was set to 78° C. and the silicon wafers were immersedinto this solution for 20 minutes. The silicon wafers used here have asize of up to 6 inches, are etched in part down to a thinness of 120 μmand have reflecting surfaces before the etching process. A scanningelectron micrograph of the surface of the silicon etched with thissolution is shown in FIG. 1, depicting a uniform and complete pyramidstructure some 6 μm in height.

[0021] A second exemplary embodiment made use of an etching solutionwhich comprises 15 l water, 600 ml 50% sodium hydroxide solution, 300 mlisopropanol and 30 ml aqueous alkaline ethylene glycol solution.Silicate was not added. This solution was heated to a temperature of 78°C. and the silicon wafers were immersed in this solution for 10 minutes.The surface of the silicon wafer structured with this etching solutionis shown in FIG. 2. A uniform pyramidal silicon surface can be seen, thepyramid having a height of approximately 3 μm.

[0022] Another exemplary embodiment made use of an etching solutionwhich contains 15 l water, 50 ml 50% sodium hydroxide solution, 100 mlisopropanol and 50 ml aqueous alkaline ethylene glycol solution,likewise without addition of silicate. This solution was also heated toa temperature of 78° C. and a subsequent etching process was performedfor 5 minutes. The silicon surface structured with this solution isshown in the scanning electron micrograph depicted in FIG. 3. A uniformand complete pyramidal structure can also be identified here. The heightof the pyramids is less than 1.5 μm.

[0023] Further silicon samples were etched with an etching solutionwhich contains 15 l water, 400 ml 50% sodium hydroxide solution, 400 mlisopropanol and 25 ml aqueous alkaline ethylene glycol solution (withoutaddition of silicate). This solution was heated to a temperature of 60°C. and the silicon sample was then etched in this solution for 10minutes. The result can be seen in FIG. 4. In this instance, the surfacealso exhibits a pyramidal structure having a height of approximately 1.5μm.

[0024] The manner in which the pyramid size decreases as the ratio ofglycol increases is apparent from exemplary embodiments 1 to 3. In the4th exemplary embodiment, a more considerably aerated aqueous alkalineethylene glycol solution is taken as a basis, for which reason it waspossible to reduce the ratio here in order to generate pyramid sizes of≦1.5 μm, as in Example 3.

[0025] Each etching solution used in the exemplary embodiments can beused immediately after production, i.e. the etching solution isimmediately capable of texturizing. A lengthy dissolution of silicon, asis necessary in the known solutions based on ethylene glycol, isomitted.

[0026] Although it is not necessary to add silicate, such an additiondoes not, however, have any disruptive effect either. This is evidentfor instance from the first exemplary embodiment in which 400 mldissolved silicate is present in the etching solution. Uniform andcomplete pyramidal structuring was also achieved in this exemplaryembodiment. The proposed etching solution contains only a small amountof isopropanol of preferably about 2 to 5 vol. % in order to minimizeisopropanol consumption.

[0027] It is also apparent from the exemplary embodiments describedabove that a precise optimization of the ratio of the individualdissolved components is not important because different concentrationsof different ratios of dissolved components make satisfactory texturesof the silicon surface possible. In other words, the process window ofthe etching solution according to the invention has a considerable size,which is particularly beneficial as regards an industrial structuringprocess. As is evident from the scanning electron micrographs of theindividual exemplary embodiments, the pyramid size can also be varied bya different solution composition while ensuring constant reliable andcomplete structuring. The process temperature is set between 60° C. and80° C. and the etching time varies between 5 and 10 minutes depending onthe desired size of pyramid.

[0028] It should also be mentioned that naturally grown oxide formedwhen the silicon wafer to be etched has been standing for several weekshas just as little disruptive effect as for example a different siliconsurface property (e.g. rough or polished).

[0029] To examine any possible influence of naturally grown oxide, waferwhich had been previously standing in ambient air for several weeks wereimmersed into the etching solution in a dry state and any wafers whichhad been standing in the water rinsing basin for about 60 minutes beforethe etching step were dipped into the etching solution in a wet state.In both instances, naturally grown oxide does not interfere with thetexture etching. A previous hydrofluoric acid dip for the removal ofthis oxide is unnecessary.

[0030] Any consumption of the etching solution can be compensated byaddition of the necessary media. Such media do not have to be especiallyheated up in order to minimize, for example, any dissolved oxygen.

[0031] The etching solution according to the invention is suitable forvarious forms of media movement. Bubblers, stirring devices, liftingapparatus or pump circulation of the solution are possible, as is acombination of such equipment.

What is claimed is:
 1. A free-standing etching solution for the wetchemical pyramidal texture etching of silicon surfaces comprising:water, an alkaline reagent, isopropanol and an aqueous alkaline ethyleneglycol solution.
 2. The free-standing etching solution as in claim 1,wherein the etching solution further comprises silicate.
 3. Thefree-standing etching solution as in claim 1, wherein the etchingsolution contains 0.5 to 5 vol % isopropanol.
 4. The free-standingetching solution as in claim 1, wherein the proportion of isopropanolpresent in the etching solution is greater than the proportion ofethylene glycol.
 5. The free-standing etching solution as in claim 4,wherein the ratio of isopropanol the ethylene glycol is at maximum 1:1.6. The free-standing etching solution as in claim 1, wherein the aqueousalkaline ethylene glycol solution is reacted with oxygen.
 7. Afree-standing etching solution for the wet chemical pyramidal textureetching of silicon surfaces comprising: water, sodium or potassiumhydroxide, isopropanol and an aqueous alkaline ethylene glycol solution.8. The free-standing etching solution as in claim 7, further comprisingsilicate.
 9. The free-standing etching solution as in claim 7, whereinthe etching solution contains 0.5 to 5 vol. % isopropanol.
 10. Thefree-standing etching solution as in claim 7, wherein the proportion ofisopropanol present in the etching solution is greater than theproportion of ethylene glycol.
 11. The free-standing etching solution asin claim 10, wherein the ratio of isopropanol to ethylene glycol is atmaximum 1:1.
 12. The free-standing etching solution as in claim 7,wherein the aqueous alkaline ethylene glycol solution is reacted withoxygen.